This colorful view of the globular star cluster NGC 6362 was captured by the Wide Field Imager attached to the MPG/ESO 2.2-m telescope at ESO’s La Silla Observatory in Chile. Credit: ESO
Globular clusters have for a long time been considered as formed of stars with similar initial chemical composition, but recently, increasing evidence has emerged regarding their more complex nature. NGC 6362 lies 25,000 light years away in the constellation Ara. It is ~13.5 billion years old and at a mass of ~50,000 solar masses, it is the least massive globular cluster with multiple detected photometric stellar populations that have different chemical abundances.
To determine the chemical composition of NGC 6362, the team used the Fibre Large Array Multi Element Spectrograph (FLAMES) mounted on one of the Unit Telescopes (UT) of the Very Large Telescope (VLT) in Chile. Using FLAMES, the researchers analyzed the spectra of over 200 giant stars and derived chemical abundances of iron and sodium in this cluster. The instrument can obtain, in one shot, high-quality spectra for about 140 stars over a field of view of about 25 arcmin diameter.
“In the case of NGC6362, we have analyzed spectra of more than 200 giant stars around the cluster in order to distinguish star members of the cluster according to their kinematics and chemistry, finally finding 160 member stars. This work is part of a larger project aimed at obtaining a complete description of this globular, in terms of photometric properties, chemical composition, kinematics,” Mucciarelli said. The study provided the first chemical characterization of NGC 6362. The found star-to-star variations in the Na+ abundance which demonstrate that this small cluster shows the same chemical anomalies observed in more massive clusters.
The research proves an important constraint that stellar clusters down to 50,000 solar masses are able to create sodium-rich stars. It is important to note that open clusters do not show these chemical anomalies and they have masses at least one of order of magnitude smaller than those of the globular clusters. Thus, thanks to the study, NGC 6362 yields an important piece of evidence toward understanding the minimum mass to properly define a globular cluster.
They noted that any theory aimed at explaining the formation and evolution of these clusters must account also for the peculiar case of NGC 6362. Moreover, the team emphasized that this collection of stars has still more mysteries waiting to be unveiled. “We are planning new observations to study the kinematics of NGC6362 in order to provide a complete characterization of its stellar populations under all the aspects, photometry, chemistry, dynamics,” Mucciarelli concluded.
http://phys.org/news/2016-04-italian-scientists-chemical-anomalies-low-mass.html
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